CHAPTER XIII. 



IRRITABILITY. 



259. We should now examine the movements of plant parts 

 in response to the influence of certain stimuli. By this 

 time we have probably observed that the direction which the 

 root and stem take upon germination of the seed is not due to 

 the position in which the seed happens to lie. Under normal 

 conditions we have seen that the root grows downward and the 

 stem upward. 



260. Influence of the earth on the direction of growth. 

 When the stem and root have been growing in these directions 

 for a short time let us place the seedling in a horizontal position, 

 so that the end of the root extends over an object of support in 

 such a way that it will be free to go in any direction. It should 

 be pinned to a cork and placed in a moist chamber. In the 

 course of twelve to twenty-four hours the root which was formerly 

 horizontal has turned the tip downward again. If we should 

 mark off millimeter spaces beginning at the tip of the root, we 

 should find that the motor zone, or region of curvature, lies in 

 the same region as that of the elongation of the root. 



Knight found that the stimulus which influences the root to 

 turn downward is the force of gravity. The reaction of the root 

 in response to this stimulus is geotropism, a turning influenced 

 by the earth. This term is applied to the growth movements of 

 plants influenced by the earth with regard to direction. While 

 the motor zone lies back of the root tip, the latter receives the 

 stimulus and is the perceptive zone. If the root tip is cut off, 

 the root is no longer geotropic, and will not turn downward 

 when placed in a horizontal position. Growth toward the earth 



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